The present invention relates to a method for the treatment of metal objects, particularly for the thermal treatment of sintered permanent magnets containing rare earth metals.
Iron, boron and neodymium form the compound Nd.sub.2 Fe.sub.14 B, which has excellent permanent-magnetic properties. The so-called NEO magnets, which are already in commercial production, are mainly composed of this compound. In addition to neodymium, the NEO magnets may also contain other rare earth metals, such as dysprosium, and in addition to iron also other transition metals such as cobalt.
In the production of NEO magnets, a cast metal alloy is crushed and pulverized, whereafter the powdered metal is compacted and sintered. After sintering, the magnets are subjected to thermal treatment at a temperature of about 600.degree. C. for about an hour in order to increase their internal coercive force. After the thermal treatment, the magnets undergo a finishing grinding. Finally, because iron and particularly neodymium are very reactive metals, these magnets must be coated either with metal or with plastic before being put into use.
In many applications a high accuracy in the dimensions of the magnets is required, and therefore the coating process must be accurately adjustable. Aluminum, which can be for instance vacuum evaporated onto the magnet surface, has been used as a coating material for NEO magnets. Although the method of vacuum evaporation, where the workpiece is not heated, is simple and widely used, it has a drawback in that the coating tends to adhere rather poorly.
The purpose of the present invention is to eliminate some of the drawbacks of the prior art and to achieve an improved and more secure method of treating sintered permanent magnets so that the magnets can also be utilized in cases where the conditions require a corrosion-proof product.